Soil columns have been used to investigate the fate and transport of explosives in different soils and environments, although there are no standardised methods [56, 80, 101, 102]. For example, soil column experiments have been used to measure the transport of individual explosives through varying soil types with differing levels of organic matter [56], as well as investigation formulations [57, 80]. At the end of the experiment, soil columns are often dissected to enable full calculation of the mass balance to account for all spiked materials and their transport pathways. This would not be possible in the real environment as contamination concentrations are unknown, and the soil is uncontained allowing far greater sideways transport. Although soil columns are not fully representative of the real environment they provide a controlled, reproducible system that can be used to predict contaminant transport rates and pathways.
Degradation
As seen in section 1.3, explosives are likely to degrade in the environment, particularly when exposed to sunlight or soil that is rich in minerals, organic compounds and microbes [103]. Degradation in soil may occur via physical mechanisms (e.g. metal ions as catalysts) or due to biodegradation. The latter can be investigated by exposing specific strains of bacteria to explosive materials at low concentrations for hours or days [104]. This is not representative of a real soil environment, therefore soil microcosms have been used where spiked soils are suspended in a basal medium to encourage bacterial growth [55, 105]. This highlights the effect of different soil types on degradation, specifically looking at characteristics such as organic carbon content, iron availability and soil texture. However, under these conditions it is difficult to separate the effect of biodegradation from physical degradation and adsorption. One way to eliminate the effect of biodegradation is to sterilise the soils. Autoclaving at 120 °C or irradiating with gamma rays destroys bacteria, but keeps the organic carbon content the same, isolating its effect [81, 104]. More destructive methods (such as incineration at 400 °C) remove both microbes and organic carbon, so generic degradation pathways in different soil types can be determined. A more representative method is to monitor the effluent from soil columns for spiked explosives and their degradation products, but such methods need to be supported by additional degradation studies if the mechanism of degradation is not understood [56, 81].
1.4.2 Computational modelling
The most important parameter in explosives’ environmental fate and transport studies is the contaminant concentration (C), which corresponds to the amount of that substance in a specific volume or mass of air, water, soil, or other material.
Many processes affect the fate, transport and transformation of a particular contaminant in the subsurface. The overall composition of leachates produced reflects the degree of aerobic and anaerobic decomposition as well as retention within the soil matrix. Understanding how this material can become mobilised and what happens to it is difficult and complex as the contaminant transport subsurface is affected by a large number of nonlinear and often interactive physical, chemical and biological processes (section 1.3) [106, 107]. Such processes can be modelled by mathematical equations, which describe the dependence of unknown quantities of interest (e.g. TNT concentration at some point and time) on known or estimable parameters (e.g. permeability, reaction coefficients, mass loading). The models are developed from conceptual models derived from laboratory and/or field observations of parameters to be interrelated. Concentration is a key quantity in fate and transport equations as a substance’s concentration in an environmental medium also determines the magnitude of its biological effect. Because of the complexity of processes, simplifying assumptions can be introduced to make the model more tractable numerically, i.e. neglecting processes considered to be of minor influence such as volatility.
The most common conceptual models in the literature are based on the assumption that solid explosive residues (ER) in soils are dissolved by infiltrating precipitation at or near the ground surface and subsequently leached through the subsurface as a dissolved phase [107, 108].
The main transport processes occurring into the subsurface are advection, dispersion and diffusion in the liquid and gaseous phases that lead to solute transport and sorption [109]; therefore advective-dispersive-reactive transport (ADRT) is the underlying process governing the migration of explosives though a porous medium [4, 109], which can be written as (1.1)
where c = concentration, [M/L3]; t = time, [T]; n = effective porosity; D = hydrodynamic dispersion, [L2/T]; z = coordinate system, [L];
The ADRT equation describes the time-varying change in concentration of reactive dissolved contaminants in saturated, homogeneous, isotropic material under steady-state and uniform flow conditions. The term nD∂2c∂z2 represents dispersive transport and nv∂c∂z corresponds to advective transport. The concentration can change if there is a different concentration elsewhere in the flowing fluid and this different concentration is carried by advection to the fixed point of interest. The concentration can also change by Fickian transport if there is a spatially varying concentration gradient in the fluid due to the dispersion. Changes in the concentration also can occur if a source or sink process, such as a chemical or biological reaction, is introducing or removing the compound of interest (Rf).
Additional physical processes in the subsurface may include capillary and gravitational forces that cause transient water flow, dissolution, volatilisation, biotransformation, abiotic reaction, and convection and conduction that result in heat transport [110]. The mechanisms of the main processes mentioned are described in the following.
Advection and dispersion processes
Advection is the most important dissolved chemical migration process active in the subsurface and reflects the migration of dissolved chemicals along with groundwater flow, i.e. the mass of actual mass movement of a fluid through the porous media. In fact, it refers to the passive movement of a solute with flowing water (or other solvent) [111].
In a porous medium, the mass flow through a unit section perpendicular